Neutral theory claims that the overwhelming majority of evolutionary changes at the molecular level are not caused by selection acting on advantageous mutants, but by random fixation of selectively neutral or very nearly neutral mutants through the cumulative effect of sampling drift (due to finite population number) under continued input of new mutations.
Matoo Kimura. (1991). The neutral theory of molecular evolution: a review of recent evidence. Jpn J Genet 66, 367-386.